1. Field of the Invention
The present invention relates to a stabilizer for mixed fuels. More particularly, the invention relates to a stabilizer for mixed fuels, which is used as a dispersing stabilizer when fine particles of coal are dispersed in a fuel oil.
2. Description of the Prior Arts
During the First World War or Second World War, investigations were made on mixed fuels comprising coal and a fuel oil such as heavy oil or crude oil for military or other applications. Each of these investigations, however, was made mainly for compensating for the shortage of petroleum, and therefore, when the supply of petroleum was stabilized, working of these investigations was stopped. On transportation of coal, unloading of coal and shifting of coal after unloading are very troublesome operations. Accordingly, a coal-incorporated liquid fuel was conceived for the purpose of reducing the handling cost of coal. In France or USA, a hydraulic transportation system for transporting powdered coal hydraulically through a pipe line was established. This system for transportation of aqueous slurries of coal was investigated also in Japan. However, coal is transported in Japan by marine transportation, and in the case of marine transportation, the transportation cost is increased when excessive water is shipped. Further, in the case of an aqueous slurry of coal, the combustion efficiency is reduced. For these reasons, the above-mentioned hydraulic transportation system was not actually worked in Japan. However, based on a concept that the foregoing defects involved in an aqueous slurry of coal will be eliminated if a mixture of coal and a fuel oil such as heavy oil or crude oil is used instead of the above-mentioned aqueous slurry, such mixed fuel, namely a coal-oil mixture (hereinafter referred to as "COM"), has recently been reconsidered. Especially, since the so-called "oil shock", the price of petroleum has drastically risen and there is a possibility that the price of COM will be reduced below the price of heavy oil. Further, coal resources are ample. Under such background, investigations on practical utilization of COM has now been promoted. In Japan, coal must be imported from abroad, and hence, reduction of the handling cost will result in reduction of the price. Therefore, it is eagerly desired to put COM into practical use. COM has the following advantages:
(I) The calorific value is higher than that of coal, and the existing combustion apparatus for petroleum can be used for COM only with a partial modification.
(II) Marine transportation by tankers is possible, and mass transportation of coal in the form of COM can be performed conveniently.
(III) Long-distance land transportation (pipe transportation) is possible.
(IV) Spontaneous combustion of coal during transportation can be prevented, and it becomes possible to import brown coal, which has not been imported though it is cheap, because spontaneous combustion is readily caused in brown coal during transportation.
(V) Storage space can be reduced and the specific gravity is higher than that of water, and even if a fire occurs, fire extinguishing can easily be accomplished in the case of COM and a flow-out accident that frequently takes place in the case of crude oil does not occur at all.
(vi) Stable supply can be assured and a high economical merit can be attained.
(vii) A large wharf or other harbour equipment is necessary for landing in the case of marine transportation of coal and a large machine or other equipment is necessary for transportation of landed coal, but COM can be landed and transported quite in the same manner as petroleum is landed and transported.
In COM, coal particles as solids are precipitated in a fuel oil as the dispersion medium owing to the difference of the specific gravity, and the precipitation speed is influenced by the viscosity of the fuel oil and the size of the coal particles. As the size of the coal particles is finer, the precipitation speed is low. Accordingly, it is desired to pulverize coal as finely as possible. However, the volume of pulverized coal is increased as the particle size becomes fine. Powdered coal now used in a power plant has such a size that 80% of the particles can pass through a 200-mesh sieve, namely a particle size of about 74 microns. Accordingly, it is expected that powdered coal having such a particle size will be used as standard powdered coal. When a surface active agent is incorporated as a stabilizer into COM, the surface active agent is absorbed in the interface between coal particles and the fuel oil and it exerts functions of disintegrating agglomerates of coal particles and preventing cohesion of coal particles. In addition to these effects, an effect of converting coal particles precipitated in the lower layer to soft precipitates is required for the stabilizer (surface active agent) for COM. Namely, it is required that when COM is stored for a long time, in order to easily retain a homogeneous state in COM by agitation, the precipitated coal particles should be in the form of very soft precipitates. After preparation, COM passes through various processes such as pipe transportation, tanker transportation and tank storage until it is actually burnt as a fuel. Accordingly, in COM it is especially important that the precipitation speed of coal particles should be low and the precipitated coal particles should be dispersed again very easily. In coal-fuel mixtures, when no stabilizer is added, substantially all of the coal particles are precipitated if the mixtures are allowed to stand for about 1 or about 2 days after preparation, though the degree of precipitation differs to some extent in these mixtures, and the resulting precipitates are very hard and the re-dispersibility thereof is very low. Accordingly, when COM is to be stored for a long time after preparation, the role of the stabilizer is very important, and development of an excellent stabilizer is eagerly desired. Since COM passes through various processes such as mentioned above after preparation, it is required that COM should be stored for at least 4 to 7 days, preferably at least 15 to 30 days and should have a good re-dispersibility after standing.
Many patent applications have been filed as regards stabilizers for COM by Petrolite Co. and others, and the majority of commercially available surface active agents are described in specifications of these patent applications. However, each of these stabilizers is still insufficient in the stabilizing effect and development of a stabilizer having a higher effect has been desired in the art. As pointed out hereinbefore, the particle size of coal in COM is relatively large, and coal particles are precipitated if COM is stored for a long time. As means for retarding precipitation of coal particles, patent specifications of Petrolite Co. disclose methods in which certain additives are added to increase the viscosity of COM, especially to impart a thixotropic property to COM. Even if these methods are adopted, however, parts of the coal particles are precipitated to form hard precipitates, and the degree of rendering COM thixotropic should naturally be limited within a certain range. Further, if COM is rendered thixotropic, the viscosity is drastically elevated, and various troubles are caused in actual applications, such as difficult handling and the practical value of COM is extremely degraded.